This is easily detectable with the proper client-side mining software. Everyone just needs to make sure that whenever they find a valid block, it is reported by the pool.
The problem is, of course, that people don't use the proper software (maybe it doesn't yet exist), or care much about trying to detect this.
If you use a PPS derivative reward scheme it doesn't matter.
If you use PPS it doesn't matter.
If you use a "PPS derivative" (by which I guess you mean *MPPS) you have the same problem, operator can hide found blocks, keep their reward to himself and misrepresent the pool's balance.
Yeah, I do like the PPS model. It's just that pools have to take so much in fees to account for the potential variance that could literally bankrupt them.
This is fixable with some protocol changes, adoption of decentralized metapools, and careful risk assessment (I'll write something about that last part pretty soon).